Digital distance measuring instrument

ABSTRACT

A distance measuring instrument comprising a high-frequency reference source for emitting a modulated light signal and from which a first and second oscillation of 1 kHz is derived in a first and a second periodic counter, respectively. A phase control means locks the phase of the emitted signal to the first oscillation. A heterodyne frequency representing the transmitter phase is applied to transpose the received signal down to 1 kHz and a second phase detector compares the phase of this signal with the second oscillation and causes additional pulses to be supplied to the first or the second counter depending on the polarity of the phase difference so as to reduce it to zero. A forward-backward counter responds in one direction to additional pulses supplied to one periodic counter and to the opposite direction to those supplied to the other periodic counter to provide a digital indication of the phase difference corresponding to the distance.

United States Patent Granqvist [45] Aug. 15, 1972 DIGITAL DISTANCEMEASURING Primary Examiner-Benjamin A. Borchelt INSTRUMENT AssistantExaminer-S. C. Buczinski [72] Inventor: Carl-Erik Granqvist, Lidingo,Swe- Attorney-Larson Taylor and Hmds den 57 ABSTRACT [73] Asslgnee: A GA Aknebolag Lldmgmsweden A distance measuring instrument comprising ahighfrequency reference source for emitting a modulated [22] Flled' Sept1970 light signal and from which a first and second oscilla- [21] Appl.No.: 76,206 tion of I kHz is derived in a first and a second periodiccounter, respectively. A phase control means Related Apphcauon Datalocks the phase of the emitted signal to the first oscil- 3 Continuationf 733 910 1 lation. Aheterodyne frequency representing the trans 1963 bd mitter phase is applied to transpose the received signal down to 1 kHzand a second phase detector compares [52] us. Cl. ..356/5 the Phase ofthis Signal with the Second Oscillation and 51 Int. Cl. ..G01c 3/08causes additional Pulses to be pp to the first or [58] Field of Search..356/5; 343/5 DP the Seeend counter depending on the P y of the phasedifference so as to reduce it to zero. A forward- 5 References Citedbackward counter responds in one direction to additional pulses suppliedto one periodic counter and to UNITED STATES PATENTS the oppositedirection to those supplied to the other 3 446 971 12/1971 Ruddock 356/5periodic counter to provide a digital indication of the phase differencecorresponding to the distance.

FOREIGN PATENTS OR APPLICATIONS 2 Cl 3 Drawing Figures 953,048 3/1964Great Britain 3- I04 kHz CALIBRATION PATH PHASE COUNTER 'j xi] MX 9PHOTOTUBE MULTIPLIER HHZ gglgggPgg FA W :1 I lkHz C W J PD l 5000 PHASEI -a 5000 kHz l DETECTOR SR lkHZ T REFERENCE (:2 l p02 PULSE 5000 l rPHASE DETECTOR SOURCE I J J1 SW5 Lei th rp A I a a i g a t it I Otrieill l 1 1 I l l" J PATENTEDAUB I 5 m2 3 5 84 3 75 SHEET 1 BF 2 ilREFERENCE M PULSE T IO3/S SOURCE '7 M SW I I I I0 lo I0 6 SR 0 L L 1ADDITIONAL COUNTER PULSE 1 SOURCE SA PC 3 SW2 I- ---2 o 2 Q/g I I I I II I- I I L PHASE COUNTER i /(CONTROL UNIT CM F 6. 1

gEESERENCE SOURCE j IIOG/S COUNTER I J I I I C Ll-LID /3 SR l IO I02 I0ADDITIONAL 6 3 1 IO /3 f COUNTER C2) i H IC) /S IJL-IL I I0 I02 I03 SA AII SW3 PHASE COUNTER \VL sw4 L I- I I o PC L5 INVENTOR CARL ERIKGRANQVIST BY (ya /2 504 ATTORNEYS PATENTEDMIGIS m2 3,684,375 sum 2 or 2LIGHT qoDuLAToR 3- l0 kHz 4 3-IO kHz MIXER Mu- J 30,00IH2 6 W HG EPHOTOTUBE MULTIPLIER u Hz \HETERODYNE GENERATOR I k Hz 0C I 5000 kHzPHASE 5000 kHz kHz COUNTERS DETECTOR SR 1 PD 2 5230 I PHASE DETECTORSOURCE I sw 5 I JL ll-l AL 1 1, /'-PC S I O I P I- 1 I I l I LPHASECOUNTER FIG. 3

nwsmon CARL ERIK GRANQVIST ny Jimm ATTORNEYS DIGITAL DISTANCE MEASURINGINSTRUMENT CROSS-REFERENCES TO RELATED APPLICATIONS This application isa continuation of US. application Ser. No. 783,910, filed Dec. 16, 1968,now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a distance measuring arrangement wherein amodulated light signal is transmitted over the distance to be measuredand the phase delay of the received modulated signal is representativeof this distance and, more particularly, to an improved arrangement ofthis type which provides a digital indication of the distance measured.

2. The Prior Art One difficulty in providing a digital indication of ameasured distance is that of providing control of the oscillationsignals. Perhaps the most common method of adjusting the phase of anoscillation is through the use of various types of adjustable reactancenetworks. In accordance with another well known method phase or timerelationships are measured with the aid of reference pulses which areapplied to a counter during the time interval to be measured. In orderto adjust the phase of an oscillation where the latter arrangement isused, it is necessary to adjust a phase-delay network such as to causethe reference pulses to be connected to the counter at a reference timecorresponding to a phase and to be disconnected in response to thesignal whose phase is to be measured. If the signal is noisy, the timeat which disconnection takes place may be influenced by the noise.

A further known phase measuring system (disclosed in British Pat. No.998,337) includes a frequency divider chain for converting an inputsignal to the chain into an output signal having a fundamental frequencylower than that of the input signal. The output signal is applied to asynchronous detector and an integrator integrates the output of thedetector. A pulse generator adds pulses to or inhibits pulses normallyoccurring at, some point on the divider chain according to whether theoutput from the integrator is respectively greater than or less than (orvice versa) a threshold level. An accumulator is used for storing thedifference between the number of pulses added and the number of pulsesinhibited.

SUMMARY OF THE INVENTION In accordance with the present invention adistance measuring instrument provided which comprises, in combination,a source of high frequency pulses of a predetermined frequency, aperiodic counter responsive to said source for producing an oscillationof a submultiple of said predetermined frequency, a second periodiccounter responsive to said source for producing a second oscillation ofsaid sub-multiple frequency, a transmitter for emitting a signal of aphase representative of said reference pulses, a receiver for receivingsaid signal after said signal has traversed the distance to be measured,the phase of the received signal representing the distance to bemeasured, and for driving from said received signal a secondary signalof the frequency of said second oscillation and representing said phase,a first phase control means for controlling the phase of said emittedsignal relative to the phase of said first oscillator, means formodifying the number of pulses supplied to the second counter relativeto the number of pulses supplied to the first counter, a second phasecontrol means responsive to the phase difference between said secondarysignal and said second oscillation for controlling said modifying meansto establish phase equality between said second oscillation and saidsecondary signal, and a forward-backward counter responsive to saidmodifying means for indicating the phase dilference between said firstoscillation and said secondary oscillation.

Other features and advantages of the invention will be set forth in orapparent from the description of the preferred embodiments foundhereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic circuit diagramof an arrangement for controlling and displaying digitally the phase ofan oscillation generally in accordance with the prior art.

FIG. 2 is a schematic circuit diagram of an arrangement for controllingand indicating the phase of a second oscillation relative to a firstoscillation, this arrangement being a modification of the circuit ofFIG. I and being preferred for incorporation into the instrument of theinvention.

FIG. 3 is a schematic circuit diagram of an instrument for measuringdistance in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a sourceof reference pulses SR produces a series of pulses at a predeterminedrate, for example, 10 pulses per second. The pulses are applied to aswitch SW1 having two outputs, one of which is coupled to the input of aperiodic counter C. The capacity of the counter C is a predeterminedvalue, for example, 1,000, so that the counter C delivers an outputpulse for every 1,000th input pulse applied thereto. For the particularvalues given, the output pulse rate of counter C will thus be 10 pulsesper second. If it is desired to provide sinusoidal signals the counteroutput may be applied to a continuous-wave generator G indicated indashed lines. Further, a feedback path, the purpose of which isdescribed below, may be provided via a switch SWA also indicated indashed lines.

The other output of SW1 is connected to the backward-stepping input of aphase counter PC, which may be a forward-backward counter ofconventional construction. Phase counter PC includes backward andforward counting inputs and a scale S which provides a net indication ofthe pulses applied to these inputs. A source of additional pulses SAprovides a series of further pulses, also at the rate of 10 pulses persecond. Source SA may, for example, simply be a delay circuit connectedto the output of source SR which produces delayed versions of the pulsesproduced by source SR, although it will be understood that other typesof pulse generators can be used. These additional pulses are applied toa second switch SW2, which is normally open but which in the closedstate thereof applies the pulses to the forward-stepping input of phasecounter PC as well as to counter C. A control unit CM provides selectiveactuation of either switch SW1 or switch SW2. It is noted that thefrequency of the pulses produced by source SA does not have to be thesame as that of the pulses produced by source SR in that application ofthe pulses from source SA is a function of the state of switch SW2 asdetermined by control unit CM.

Considering the operation of the embodiment of FIG. 1, for the normalstates of switches SW1 and SW2 shown, the reference pulses from sourceSR are applied to counter C, which, as described hereinabove forspecific example given, delivers an output pulse for every 1000th inputpulse applied thereto. To advance the phase of the output oscillationfrom counter C, switch SW2 is actuated, which causes one or more pulsesfrom source SA to be applied to counter C in addi tion to the referencepulses applied from source SR. These additional pulses are also appliedto phase counter PC in the forward direction so that each pulse advancesthe phase counter PC by one step. Because of the additional pulsesapplied to counter C, the output pulse provided thereby is advanced inphase by an amount corresponding to the number of additional pulsessupplied by source SA, provided, of course, that these latter pulses areseparated in time from the reference pulses so that the counter C willrespond to them separately. For the example given, i.e., where source SAproduces pulses at a pulse per second rate, if each one of 500 referencepulses produced by source SR is followed by an additional pulse producedby source SA, the counter C will reach the 1,000 count at which anoutput pulse produced after in only half its normal period, thuscorresponding to an output phase shift of 180.

As pointed out hereinabove the additional pulses must be separate intime from the reference pulses in order to ensure that the counter Cresponds separately thereto. Separation of these two sets of pulses maybe accomplished by well known prior art techniques. For example, asmentioned, source SA may merely comprise a delay circuit for producingdelayed versions of the reference pulses, which delayed pulses areutilized as the additional pulses, as indicated in FIG. 1. Further, theoutput pulses of counter C fed back by closing switch SWA may be used toprovide additional pulses.

A phase retardation is obtained where control unit CM is used to actuateswitch SW1 to remove one or more reference pulses from the input circuitof counter C and to apply these pulses to the backward-stepping input ofphase counter PC. The amount of phase retardation is registered by phasecounter PC and indicated on scale S.

To summarize the operation described hereinabove, phase counter PCcounts forwardly or backwardly as the number of pulses fed to counter Cis increased or decreased relative to a normal rate and thereby providesa digital display of the phase of the output oscillation from counter C.

A similar system to that shown in FIG. 1 is disclosed in British Pat.No. 998,337 referred to above.

Referring to FIG. 2, there is shown a schematic circuit diagram of asystem wherein the phase of one oscillation is controlled relative to asecond oscillation. The circuit of FIG. 2 is similar to that of FIG. Iand like elements therein have been given the same reference letters. InFIG. 2, the output of a source SR is connected to the inputs of firstand second counters C and C2, each of which operates as described inconnection with FIG. I. An auxiliary source SA is connected through aswitch SW3 to an additional input of counter C and to thebackward-stepping input of phase counter PC. Similarly, the additionalpulses produced by source SA are applied to a second switch SW2 forapplication to a second input of counter C2 and to the forward steppinginput of phase counter PC.

In operation, closing of SW3 causes the application of the additionalpulses to counter C, which pulses are counted backwardly by phasecounter PC. Closing of SW4 causes pulses to be applied to counter C2 andto the forward input of phase counter PC, to thereby provide a displayof the phase of the output oscillation from counter C2 relative to thatof counter C.

Referring to FIG. 3 there is shown an embodiment of the inventionincorporated in a distance measuring instrument of the type which emitsa modulated light beam and compares the modulation phase of a reflectedbeam with that of light which has been passed to the receiver over acalibration path provided in the instrument. The embodiment of FIG. 3includes a phase adjusting and indicating arrangement generally denotedA and enclosed within dashed lines. This arrangement is similar to thatshown in FIG. 2 like elements have been denoted by the same referenceletters as previously used. In the embodiment of FIG. 3, light from alight source LS is passed through a light modulator LM such as a Kerrcell and is transmitted as a modulated beam via an optical systemcomprising a mirror MI. The reflected or returned beam is directed by asecond mirror M2 to a phototube PT. The modulation signal for the Kerrcell is provided by a frequency multiplier MU which, in a specificexample, receives a 5,000 kHz signal from a source of reference signalSR and which multiplies this signal by 6. The output voltage of 30,000kHz produced by multiplier MU is applied to light modulator LM. Acontrol grid of phototube PT is modulated by a 30,001 kHz voltagedelivered by a heterodyne generator I-IG so that the output voltage ofphototube PT has a frequency of 1 kHz.

The outputs of heterodyne generator HG and of multiplier MU are appliedto a mixer MX which yields a 1- kHz output voltage. The signal producedby mixer MX is applied to a first phase detector PD which produces aphase control voltage in response to the phase difference between thevoltages applied to two inputs thereof and which controls the phase ofheterodyne generator I-IG. A second phase detector PD2 is pro vided tocompare the phase of the output signal of phototube PT with that of theinput signal supplied to the other input of phase detector PD2.

As stated hereinabove, the arrangement denoted A in FIG. 3 issubstantially the same as that shown in FIG. 2. In FIG. 3, pulses fromthe outputs of counters C and C2 are applied to first and secondswitches SW5 and SW6, respectively. Switch SW5 is connected to a secondinput of counter C and to the backwardstepping input of a phase counterPC, whereas the output of switch SW6 is connected to an input of counterC2 and to the forward-stepping input of phase counter PC. The switchesSW5 and SW6 are controlled by outputs from PD2 such that switch SW5 isopen only if the phase difference between the signals applied to PD2 isnegative and switch SW6 is open only if this phase difference ispositive. The second inputs of phase detectors PD and PD2 are connectedto the outputs of counters C and C2, respectively.

In the operation of the system of FIG. 3, a modulated light beam fromlight source LS passes either over the distance to be measured, asindicated in the drawing, or, if the-mirrors M1 and M2 are removed, viaan adjustable calibration path CP to phototube PT. Preliminarily,calibration path CP is adjusted to make the phase of the emitted beamequal to the output phase of counter C and the phase of the receivedbeam equal to output phase of counter C2. Phase counter PC is then setat a zero reading and the instrument is prepared for measuring thedistance in question, mirrors M1 and M2 being placed as shown in FIG. 3.It will be appreciated that the received beam has a delay correspondingto the distance to be measured. At this time, the phase of the emittedbeam is still equal to the phase of the output from counter C. Phasecounter PC is switched into operation and the phase of counter C2relative to counter C is adjusted by means of additional pulses throughswitches SW5 and SW6 in the positive or the negative direction untilthere is phase equality between the inputs to phase detector PD2. Thereceived signal from phototube PT is then in phase with the output fromcounter C2, and the existing phase difference is displayed on phasecounter PC, which can he graduated in appropriate units of length suchas meters.

It will be appreciated that if two signals of equal frequency anddifferent phases are both heterodyned with one and the same frequency,then the transposed frequencies also have the same phase difference.Such heterodyning takes place in the present instrument wherein theemitted beam is modulated with a 30,000 kHz signal and the phasemeasurement takes place at 1 kHz. The low frequency signal whose phaserepresents the emitted beam is derived from mixer MX, wherein the 30,000kHz oscillation modulating the emitted beam is heterodyned with 30,001kHz from heterodyne generator HG. The first phase detector PD comparesthe output phase from mixer MX with that from counter C and derives aphase control voltage which adjusts the phase of heterodyne generator HGand reduces the phase difference between the two inputs of phasedetector PD to zero.

With the beam passing over the calibration path CP and phase counter PCreset to zero, second phase detector PD2 compares the phases at the twoinputs thereof and actuates either switch SW5 or switch SW6 until thephase difference between the outputs from counters C and C2 equals thatbetween the outputs from mixer MX and phototube PT, that is, correspondsto the delay over the calibration path.

This process is repeated in the same general way with the beam passingover the unknown distance except that phase counter PC is then activatedso that every adjustment of the phase difference is registered by acorresponding count on phase counter PC. When there is no phasedifference between the inputs to second phase detector PD2, no signalexists at either the negative or the sitive output thereof and the phasedifference be ween the outputs from counters C and C2 corresponds to thedifference in time delay over the unknown distance and over thecalibration path CP.

Although the invention has been described in detail with particularreference to preferred embodiments thereof, it will be understood thatvariations and modifications can be efiected within the scope and spiritof the invention as described hereinabove and as defined in the appendedclaims.

lclaim:

1. A distance measuring instrument comprising, in combination: a sourceof high-frequency reference pulses of a predetermined frequency, a firstperiodic counter responsive to said source for producing a firstoscillation of a submultiple of said predetermined frequency, a secondperiodic counter responsive to said source for producing a secondoscillation of said submultiple frequency, a transmitter for emitting asignal of a phase representative of said reference pulses, a receiverfor receiving said signal after said signal has traversed the distanceto be measured, the phase of said received signal representing thedistance to be measured, and for deriving from said received signal asecondary signal of the frequency of said second oscillation andrepresenting said phase, a first phase control means for controlling thephase of said emitted signal relative to the phase of said firstoscillation, means for modifying the number of pulses supplied to saidsecond counter relative to the number supplied to said first counter, asecond phase control means responsive to the phase difference betweensaid secondary signal and said second oscillation for controlling saidmodifying means to establish phase equality between said secondoscillation and said secondary signal, and a forwardbackward counterresponsive to said modifying means for indicating the phase differencebetween said first oscillation and said secondary oscillation.

2. An instrument as claimed in claim 1, in which said modifying meanscomprises a first switch for supplying additional pulses to said firstcounter, and to said forward-backward counter for actuating saidforwardbackward counter in the forward direction and a second switch forsupplying additional pulses to said second counter, and to saidforward-backward counter for actuating forward-backward counter in thebackward direction.

1. A distance measuring instrument comprising, in combination: a sourceof high-frequency reference pulses of a predetermined frequency, a firstperiodic counter responsive to said source for producing a firstoscillation of a submultiple of said predetermined frequency, a secondperiodic counter responsive to said source for producing a secondoscillation of said submultiple frequency, a transmitter for emitting asignal of a phase representative of said reference pulses, a receiverfor receiving said signal after said signal has traversed the distanceto be measured, the phase of said received signal representing thedistance to be measured, and for deriving from said received signal asecondary signal of the frequency of said second oscillation andrepresenting said phase, a first phase control means for controlling thephase of said emitted signal relative to the phase of said firstoscillation, means for modifying the number of pulses supplied to saidsecond counter relative to the number supplied to said first counter, asecond phase control means responsive to the phase difference betweensaid secondary signal and said second oscillation for controlling saidmodifying means to establish phase equality between said secondoscillation and said secondary signal, and a forwardbackward counterresponsive to said modifying means for indicating the phase differencebetween said fiRst oscillation and said secondary oscillation.
 2. Aninstrument as claimed in claim 1, in which said modifying meanscomprises a first switch for supplying additional pulses to said firstcounter, and to said forward-backward counter for actuating saidforward-backward counter in the forward direction and a second switchfor supplying additional pulses to said second counter, and to saidforward-backward counter for actuating forward-backward counter in thebackward direction.